In order to provide improved communication services and increased efficiency, cellular communication systems are continuously developed and enhanced. Currently, the 3rd Generation Partnership Project (3GPP) standards body is in the process of standardising improvements to the Universal Mobile Telecommunication System (UMTS) known as Long Term Evolution (LTE).
LTE is an example of a cellular communication system which employs a plurality of different bandwidths. Specifically, LTE is an Orthogonal Frequency Division Multiplex (OFDM) cellular communication system where the individual OFDM subcarriers have a fixed bandwidth and a different number of subcarriers is used for different bandwidths. LTE supports operation with a bandwidth of 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz and 20 MHz with 72, 144, 288, 600 and 1200 active 15 kHz subcarriers respectively (the remaining subcarriers are used as a guard band).
In a variable bandwidth OFDM cellular system such as 3GPP Evolved UTRA/LTE, the base stations transmit reference signals containing cell-specific reference sequences. The reference signal and sequence is used by the user equipments to detect the presence of specific base stations. In addition, the base stations transmit broadcast control channels (BCH) that among other things include data identifying the specific bandwidth used by the base station transmitting the broadcast control channel.
When a user equipment (UE) is switched on it proceeds to search for available base stations. This initial search is based on the user equipment trying to detect the presence of any reference signals and sequences and on the decoding of broadcast control channels for any detected reference signals.
In some systems, such as LTE, the initial cell-search process must be performed without the user equipment having any information of which bandwidths are actually used by the base stations. In LTE the user equipment may specifically perform the entire cell search process based on only on the narrowest bandwidth, i.e. it may exclusively use signals within a 1.25 MHz window for all the steps of the initial cell search regardless of the actual bandwidth used by the present base stations. This approach relies on the broadcast control channel for higher bandwidths being limited to a 1.25 MHz window corresponding to the narrowest bandwidth in use.
In order to ensure a robust and high performance cell search based on the reference signals and sequences, it is important that the reference sequences have properties which are particularly suitable for detection and estimation. Accordingly, the reference sequences should be selected as sequences that have strong auto correlation and cross correlation properties.
It has been proposed to base the reference sequences on Generalized Chirp-Like (GCL) sequences. GCL sequences have a number of attractive qualities that make them particularly suitable for use as reference sequences in systems such as LTE. Specifically, GCL sequences have very good auto-correlation properties and cross correlation properties between different GCL sequences of the same length. In addition, GCL sequences have constant amplitude thereby making them suitable for transmission as part of a reference signal.
A further description of GCL sequences may be found in e.g. United States of America patent application US2005/0226140 A1.
In systems such as LTE, the number of symbols in the reference sequence depends on the bandwidth used. Specifically, LTE employs a frame structure comprising 7 time slots with the reference signal being transmitted in the first time slot which has every sixth subcarrier allocated for transmission of a reference signal symbol. Thus, LTE has 12, 24, 48, 200 and 200 reference signal subcarriers or reference sequence symbols for the respective bandwidths of 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz and 20 MHz.
In order to apply the GCL sequences to LTE systems, sequences with different sequence lengths must be generated depending on the specific bandwidth used. However, this has associated disadvantages. For example, searching for GCL sequence of a higher bandwidth within a 1.25 MHz window is less reliable as the GCL properties are based on full length sequences. As GCL sequences are optimized for the bandwidth they are designed for, a user equipment not having information of the specific bandwidth used must search for all GCL sequences. However, such an approach is time and resource demanding thereby resulting in suboptimal cell search and acquisition performance.
Hence, an improved cellular communication system would be advantageous and in particular a system allowing increased flexibility, reduced complexity, reduced resource demand, reduced cell search/detection/acquisition delay, improved suitability for operation with different bandwidths and/or improved performance would be advantageous.